Nanowires have diameters (or principal cross-sectional thickness dimensions) of tens or hundreds of nanometers and exhibit high aspect ratios (length-to-diameter ratio), for example aspect ratios of 100 to 1000 or more. They are considered to approximate one-dimensional material shapes and they often exhibit mechanical or electrical or thermal properties not observed in bulk or three-dimensional materials of like composition.
Metal nanowires have utility in various electronic and microelectronic applications and various metal compositions have been considered for formation as nanowire structures. Nanowires, in common with nanoparticles also have the useful property of presenting a very high surface to volume ratio, a characteristic which may be advantageous in battery electrodes.
Metallic nanowires have been produced using a variety of carefully-performed and often high-cost approaches. Some current nanowire fabrication processes include photolithography, vapor-solid-liquid growth with patterning catalyst, and using templates such as anodized aluminum oxide.
One approach is described in U.S. Pat. No. 6,841,013 and its two continuation patents, each assigned to the assignee (or its designee) of this invention. The '013 patent process comprises the deposition on a substrate of a thin film (for example, about two micrometers thick) comprising a metal phase dispersed as small volumes in a host matrix phase. The film is deposited so that it displays an elevated compressive stress, and the matrix phase maintains the dispersed metal bodies under stress in a dispersed, but diffusible state. The disclosed matrix phase materials are metal nitrides, carbides, oxides, borides, and carbon based materials which may be formed in the deposition process and which immediately apply stress on the dispersed bodies of metal phase. The composite deposited film self-relieves its compressive stress by emission of metal phase material from the free surface of the film as metal nanowires. When the initial compressive stress is relieved and the film has “relaxed,” the emission of nanowires slows and ceases. A sheath or cap may be applied over the deposited film to delay the emission of nanowires. Nanowire emission may then be encouraged by removing the sheath or by scratching or piercing the sheath to permit nanowire growth through it. The formation of bismuth nanowires is illustrated.
It would be advantageous to develop a method of preparing metal nanowires from thin films that permits more control and flexibility in the cross-sectional size of metal nanowires and in the timing and rate at which they may be produced.